1. Field of the Invention
The present invention relates to an electric power steering system.
2. Description of Related Art
In an electric power steering system, torque assistance is provided to a steering operation by transmitting the rotation of a rotary shaft of an electric motor to a worm constituting a speed reducer via a joint, reducing the speed of the rotation with the use of a worm wheel engaging with the worm, and transmitting the rotation, whose speed has been reduced, to a steered mechanism. A backlash is required for engagement between the worm and the worm wheel, and thus, there is a possibility that gear rattle due to the backlash may be generated at the time of traveling.
Thus, conventionally, an electric power steering system has been proposed in which a backlash is removed by elastically oscillating and urging a second bearing that supports one end of the worm toward the worm wheel with the use of an urging member, around a first bearing that supports the other end (motor-side end) of the worm. An electric power steering system has also been proposed in which paired elastic members are arranged on respective sides of an inner ring of a first bearing so as to suppress the backlash of the bearing. For example, such an electric power steering system is disclosed in Japanese Patent Application Publication No. 2012-101649 (JP 2012-101649 A).
It has also been proposed that, in a worm support device in which a worm is oscillated and urged toward a worm wheel and the worm is urged in an axial direction, a curvature radius of a raceway groove of a second bearing is set to be greater than or equal to 52.5% of the diameter of a ball. Such a worm support device is disclosed in Japanese Patent Application Publication No. 2004-301263 (JP 2004-301263 A).
When the urging load of the urging member is increased, the increase in the urging load can contribute to suppression of gear rattle, but steering feeling during fine steering is deteriorated due to increase in frictional resistance at the time of start of movement of the worm in the axial direction during the fine steering. To the contrary, when the urging load of the urging member is decreased, steering feeling during fine steering is improved due to decrease in the frictional resistance at the time of start of movement of the worm in the axial direction during the fine steering, but the gear rattle cannot be suppressed.